ECG Interpretation: All you need to know

Approach all EKGs systemically. Always note: rate, rhythm, QRS axis, complexes and intervals, chamber enlargement, ischemia/infarction, compare with prior EKG.

ECG Cheat sheet
ECG Cheat sheet

Rate (atrial, ventricular)

  • If the rhythm is regular, use the counting method (300 / # large boxes).
300 Method for Determining Heart Rate on ECG
300 Method for Determining Heart Rate on ECG
  • If the rhythm is irregular, count R waves in the rhythm strip and multiply by 6 (EKG printoutsrecord 10 seconds)
  • Normal 60-100bpm; <60bpm is bradycardia, >100bpm is tachycardia

Rhythm (regular or irregular; sinus vs. non-sinus)

  • Sinus rhythm defined as: P before every QRS, regular w/ rate 60-100, P wave upright in Leads I, II, aVF, V5-6
  • P waves/morphology: Determine :
    • If a P wave is present (best leads to visualize P wave are II and V1)
    • The atrial rate (100-180 : sinustachycardia; 140-220 : atrial tachycardia, AVNRT, AVRT; 260-320 : atrial flutter)
    • Axis (P wave upright in II and biphasic in V1)
  • QRS morphology:
    • Narrow (<120ms) – supraventricular rhythm.
    • Wide (>120ms) – aberrant supraventricular conduction or ventricular origin
  • P wave/QRS complex association:
    • If not 1:1 association, determine if number of P>QRS (AV block) or P<QRS (accelerated junctional or ventricular rhythm).
    • If P precedes QRS, evaluate the PR interval. If P after QRS, evaluate the RP interval and determine if PR or RP interval is fixed or variable

QRS Axis

  • Normal axis (-30 to 90º): QRS complex is positive (upright) in leads I and II
  • Leftward AxisLeft Axis Deviation(LAD) (-30 to -90º): QRS complex is positive in lead I but negative in lead II
    • Differential Diagnosis of Leftward Axis (Left Axis Deviation):
      • normal variant
      • mechanical shifts
      • LVH (left ventricular hypertrophy)
      • LBBB (Left bundle branch block)
      • LAFB (Left anterior fascicular block)
      • congenital heart disease
      • emphysema
      • hyperKalemia
      • ventricular ectopic rhythms
      • WPW (Wolff-Parkinson-White)
      • inferior MI
  • Rightward axisRight Axis Deviation (RAD) (90 to 180º): QRS complex is negative in lead I and positive in leads II, aVF
    • Differential Diagnosis of Rightward Axis Right Axis Deviation (RAD):
      • normal variant
      • mechanical shifts
      • RVH (Right Ventricular Hypertrophy)
      • LPFB (Left Posterior Fascicular Block)
      • dextrocardia
      • ventricular ectopic rhythms
      • WPW
      • lateral MI
      • (NB: RBBB rarely causes RAD)

  • Extreme axis deviation/northwest axis (180 to -90º): QRS complexes negative in both I and II
    • Differential Diagnosis of Extreme axis deviation/northwest axis:
      • Lead transposition
      • ventricular ectopic rhythms
      • hyperkalemia
      • artificial pacing
      • severe RVH (Right Ventricular Hypertrophy)
  • Clockwise/counterclockwise rotation (i.e. “R wave progression”): R wave amplitude typically increases from V1 to V5, with transition of R>S in amplitude at V3 or V4.
    • Counterclockwise transition occurs prior to V3 due to:
      • RVH (Right Ventricular Hypertrophy)
      • WPW
      • LAFB
      • posterior MI
    • Clockwise transition occurs after V4 due to :
      • cardiomyopathy
      • LVH
      • LBBB
      • anterior MI
    • Both Clockwise and counterclockwise rotation are nonspecific and can be normal (Am Heart J 2004;148:80)
  • Low voltage: Average QRS amplitude < 5 mm in I, II, III and < 10 mm in precordial leads
    • Differential Diagnosis of Low voltage QRS complex:
      • obesity
      • pericardial effusion
      • pneumothorax
      • COPD
      • restrictive or infiltrative CM (particularly amyloidosis)
      • severe hypothyroidism
      • anasarca

Complexes and Intervals

P wave: Right and left atrial depolarization; normal duration <120ms

PR interval:

  • Atrial depolarization, AV node and His-Purkinje conduction.
    • Normally 140-200ms, changes with rate (shortened at faster rates, longer at lower rates – due to autonomic effects on AV nodal conduction)


  • Ventricular depolarization.
  • Normal duration 60-100ms, not influenced by Heart Rate.
  • QRS 100-120ms is seen with incomplete BBB or interventricular conduction delay (IVCD);
  • QRS > 120ms represents:
    • BBB (Bundle Branch Block)
    • ventricular activation (PVC, VT, fusion beats, WPW, paced beats)
    • hyperKalemia
    • Na channel poisoning
    • aberrancy
    • hypothermia
  • RBBB:
    • QRS > 120
    • V1 with rSR’
    • V6 with wide qRS
    • Lead I will show a shallow broad S
RBBB (right bundle branch block)
RBBB (right bundle branch block)
  • LBBB:
    • QRS > 120 mm
    • V1 with wide negative QS
    • V6 with wide tall R
LBBB (left bundle branch block)
LBBB (left bundle branch block)

  • LAFB:
    • Left axis deviation (-45 to -90) w/ QRS < 120
    • qR in I, aVL
    • rS in II, III, aVF
    • Common, nonspecific
  • LPFB:
    • Right axis deviation (0 to +90) w/ QRS < 120
    • No alternate reason (e.g., RVH, emphysema, lateral MI, PE)
    • Rare to see in isolation, usually occurs with RBBB
  • Bifascicular block: RBBB with either LAFB or LPFB

ST segment: Represents a time of electrical silence. See “Ischemia/Infarction


  • Ventricular repolarization, with a slow upstroke and a rapid return to the isoelectric line after peaking.
  • Usually asymmetric and in the same direction as the QRS. Should have smooth contours (bumps in T are usually buried P waves)

U wave:

  • occurs in the same direction as T wave
  • rate-dependent (shorter at faster rates)
  • differential diagnosis: bradycardia, hypoKalemia / Mg /Ca , hypothermia

QT interval:

  • Ventricular depolarization and repolarization
  • Excludes U-wave unless fused with the T wave
  • Rate-dependent (shortened at faster rates)
  • Normal <440ms in men, <460ms in women

Chamber Enlargement

  • LVH:
    • Sokolow-Lyon criteria: S in V1+ R in V5 or V6 ≥35mm OR R in aVL ≥11mm.
    • Cornell criteria: S in V3+ R in aVL > 28mm (men) or 20mm (women)
  • RVH: R>S or R ≥7mm in V1, S ≥7mm in V5 or V6
  • LAE: negative p wave in V1 >1mm wide and deep, total p wave duration >110ms in II
  • RAE: p wave >2.5mm in lead II

READ MORE about Left Ventricular Hypertrophy (LVH): How to Recognize it on ECG [With Examples]


  • Analyze abnormalities along the vectors of ventricular depolarization and repolarization (QRS-ST-T)
  • T-wave abnormalities:
    • Hyperacute, symmetric T-waves can be found within minutes; followed by T wave inversions (≥0.1 mV in 2 contiguous leads)
  • ST depression:
    • Suggests subendocardial injury;
    • ≥0.05 mV below the baseline (PR segment), measured at the J point, in two contiguous leads, downsloping or horizontal = more ominous.
    • ST depressions do not localize to territories (Circ Res 1998;82;957) NB: always look for ST elevations to rule out reciprocal ST depression.
    • Digoxin toxicity: scooping ST depressions.
  • ST elevation:
    • Suggests transmural ischemia;
    • ≥0.1 mV, except for leads V2 to V3 (≥0.2 mV in men ≥40yo and ≥0.15 mV in women), measured at the J point.
    • PR segment is the isoelectric interval on the ECG and can be used to assess ST segment elevation/depression.
Coronary Artery Territory Supply and ECG lead Changes
Coronary Artery Territory Supply and ECG lead Changes

READ MORE: Know the Differential for ST Segment Elevation, It’s More Than Just ACS

ST Segment Elevation - Differential Diagnosis
ST Segment Elevation – Differential Diagnosis

  • Q-wave:
    • Usually a marker of scar;
    • Must be deep (>1 mm) and broad (>0.04 seconds), more likely prior MI if inverted T wave in same lead.
    • Pathologic Q wave defined by
      • 40ms duration (1 box wide),
      • 25% height of QRS.
    • “Isolated Q in III is free” (non-pathologic).
  • Sgarbossa Criteria:
    • Used to diagnose acute MI in presence of LBBB. Score of 3 = 90% Sp
    • Concordant STE > 1mm in any lead = 5 points;
    • Discordant STE > 5 mm in any lead = 2 points;
    • ST depression > 1 mm in V1- V3 = 3 points.
Sgarbossa Criteria
Sgarbossa Criteria
  • Wellens’ Syndrome:
    • Sign of critical prox LM (Left Main) or proximal LAD lesion, 75% MI in <2wks.
    • Patient often pain free with history of angina.
    • Normal/slightly elevated troponin.
    • Symmetric, deeply inverted T waves or biphasic T waves in V2 + V3.
    • Isoelectric or minimally elevated (<1mm) ST segment.
    • No precordial Q waves. (Am J Emerg Med 2002;20:7)

Electrolyte Abnormalities

Characteristic ECG Findings in Electolyte Abnormalities
Characteristic ECG Findings in Electolyte Abnormalities

J-point Elevation Syndromes

Early repolarization:

  • ST segment elevation in absence of chest pain, terminal QRS slur, or terminal QRS notch
  • Features suspicious for malignant forms of ER:
    • 1) Family history of sudden cardiac arrest or early unexplained death.
    • 2) evalution and workup suggestive of a channelopathy.
    • 3) history of unheralded syncope suggestive of an arrhthmogenic pathogensis (Circ 2016; 133:1520)

Brugada Syndrome (Circ Arrhythm Electrophys 2012;5:606)

  • Autosomal dominant, SCN5A loss of function mutation in 10-30% pts
  • Male > Female, more common to have nocturnal cardiac arrest
  • p/w VT/VF or sudden cardiac death
Brugada Syndrome Type 1 - Coved type ST segment elevation
Brugada Syndrome Type 1 – Coved type ST segment elevation

Osborn Wave

  • Hypothermia, T<93 F
  • Elevation of J point height roughly proportional to degree of hypothermia (n.b. neg in V1 & aVR)
Osborn Wave ECG
Osborn Wave ECG

Epsilon Wave

  • Found in ARVC (Arrhythmogenic right ventricular cardiomyopathy)
  • Most specific in V1 (30% w/ ARVC)
  • Low frequency, positive terminal deflection in V1-V3
Epsilon Wave ECG
Epsilon Wave ECG

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